Biologically derived fuels, such as triglycerides and methyl ester fatty acids are commonly used in diesel engines as a renewable hydrocarbon fuel. Unfortunately, these biologically derived fuels have certain physiochemical properties that make them unsuitable for use in cold weather. Biodiesel, a renewable fuel composed of mono-alkyl esters of long chain fatty acids, has the advantages of lower cloud- and gel-points compared to vegetable derived oils, but still experiences gelling in cold weather. Notably, even with an anti-gelling agent, the viscosity of biodiesel is too high for use in aviation and spark ignition engines.
Biodiesel is typically produced from oils or fats using transesterification or acid esterification. The transesterification process reacts an alcohol (e.g., methanol) with the triglyceride oils contained in vegetable oils, animal fats, or recycled greases, forming mono-alkyl esters of long chain fatty acids (biodiesel) and glycerin (soap). The reaction requires heat and a strong base catalyst, such as sodium hydroxide or potassium hydroxide. Some feedstocks must be pretreated before undergoing transesterification. For example, feedstocks with more than 4% free fatty acids, which include inedible animal fats and recycled greases, will typically be pretreated in an acid esterification process to lower the weight percent of free fatty acids. In the pretreatment process, the feedstock is reacted with an alcohol, such as methanol, in the presence of a strong acid catalyst, such as sulfuric acid, which converts the free fatty acids into biodiesel.
There are processes that use pyrolysis and catalysts to improve the physiochemical properties of mono-alkyl esters; however, these processes typically result in many undesirable byproducts.